It’s very possible. One in five seafood samples tested worldwide turns out to be completely different from what the menu or packaging says, according to a report on seafood fraud released Wednesday by the ocean conservation group Oceana. Of the more than 25,000 seafood samples the group analyzed, 20 percent were incorrectly labeled.

“It is likely that the average consumer has eaten mislabeled fish for sure,” said Beth Lowell, the senior campaign director for Oceana and an author of the paper. “You’re getting ripped off, while you enjoyed your meal you’re paying a high price for a low fish.”

The biggest impostor, fittingly, was farmed Asian catfish, a fish with white flesh that is easily disguised when it’s filleted and drenched in sauce. It was sold in place of 18 types of more expensive fish, including perch, cod and grouper.

The report is a sort of meta-analysis of more than 200 studies from 55 countries. One of those studies found that in Italy, 82 percent of the 200 perch, groupers and swordfish sampled were mislabeled. King mackerel, which is high in mercury, was sold as “barracuda” and “wahoo” in South Africa. In Hong Kong, only one out of 29 samples of “abalone” was correctly labeled. Two sushi chefs in Santa Monica in Southern California were charged with selling endangered whale meat as fatty tuna.

Using the various studies, the researchers created an interactive map that shows where they found cases of phony fish. The studies include DNA analyses from peer-reviewed papers, newspaper investigations and about 10 of Oceana’s own studies. The report found examples of mislabeling at every level of the seafood supply chain, including the wholesaler, the importer and the retailer.

“We kept thinking we’d find a success story, a place where seafood wouldn’t be mislabeled,” Ms. Lowell said. “Every single study that we reviewed except for one found seafood fraud.”

Even that case had a caveat Ms. Lowell said, because it took place in Tasmania where some mislabeling, like calling hake “smoked cod,” is allowed under Australian regulation.

About 80 percent of the studies were conducted at grocery stores and restaurants. Because these locations are at the end of the supply chain, retailers tend to have higher instances of mislabeling. The report doesn’t name names. Ms. Lowell said the researchers were not sure whether the restaurants and food stores knowingly deceived their customers, or if they themselves were victims of a bait and switch when they purchased the fish.

They found that 58 percent of the mislabeled samples were substituted for fish that could potentially pose health risks to certain consumers, especially pregnant women and children. In a New York grocery store they found blueline tilefish, which is on the Food and Drug Administration’s “Do Not Eat” list because of its high mercury, sold as “Alaskan halibut” and “red snapper.”

In some cases the substituted fish turned out to be an endangered species, such as in Brazil where “sharks” were actually meat from the largetooth sawfish, which is critically endangered according to the International Union for Conservation of Nature.

Ms. Lowell said that one way to combat seafood fraud in the United States is by implementing stricter regulations for fish-tracking that extend throughout the supply chain.

Gavin Gibbons, the spokesman for the National Fisheries Institute, a trade group that represents the seafood industry, criticized the report and stressed that its findings reflected only what the selected studies found and were not representative of all seafood sold globally. He added that the best fix for seafood fraud is more enforcement of the law rather than more bureaucratic regulations.

“Oceana’s focus on the most often mislabeled species distorts their findings by design,” he said in an email.

Ms. Lowell disagreed. With more than 25,000 samples tested from around the world, she said it is the most comprehensive review of seafood fraud to date.

“This report reveals that it’s a global problem,” she said, “and it’s not going to go away on its own.”